Given the recent heavy rainstorms, ponding and other weather-related problems that have resulted, a number of fields and areas of fields across Iowa have some sorry-looking corn stands. Farmers are considering replanting these fields and larger areas of fields to soybeans. But before you decide to replant cornfields to beans, two things must be resolved.
• How will you remove the existing corn stand?
• Was there a residual herbicide treatment applied to the corn?
Mike Owen, an Iowa State University Extension weed management specialist, says removing the existing corn stand can be accomplished several ways.
What herbicide was applied?
First, you can use glyphosate herbicide if the corn hybrid is not resistant to glyphosate. Or you can use tillage.
Second, replanting to soybeans may be more of a problem than you realize if one of the herbicides used has a rotational restriction that prohibits replanting to soybeans. "It is critically important that you know what was applied to the corn field and check the label to make sure that soybeans are a replant option," says Owen.
While farmers may rationalize that a herbicide (for example, atrazine) was used at a low rate and that the amount of rain has likely reduced the potential for injury to the rotational crop, it is important to follow the label. In many cases, this type of rationalization will result in serious damage to the soybean crop, he notes.
Check the rotational interval
Owen has put together two tables listing some of the herbicide options available to control existing corn stands and the rotational intervals for replanting to soybeans. "Note that these are only an overview and you need to check the specific herbicide label prior to any replant decision," he emphasizes.
To view the information in the tables and for other advice on replanting options, go to www.extension.iastate.edu/CropNews/2008/0531MikeOwen2.htm
Getting back to the other question--if the previously applied herbicide requires that corn be replanted, how do you kill the existing poor stand of corn?
Kill an existing poor stand
If the corn hybrid is a known herbicide resistant cultivar, your options are somewhat limited. If the hybrid was Roundup Ready, Owen says the use of Liberty or paraquat aren't likely to consistently control the emerged corn.
If on the other hand, the cultivar was not Roundup Ready, you have an excellent option with use of glyphosate, he adds. But keep in mind that some corn hybrids are genetically engineered to be resistant to both glyphosate and Liberty. Check with the seed company to make sure the hybrid you have is not resistant to multiple herbicides.
In cases where the hybrid is a Roundup Ready cultivar or a hybrid with multiple herbicide resistance, Owen says you are limited to tillage or the use of Select Max. Select Max herbicide has a supplemental label for control of corn in replant situations, but requires a 6 day interval between application and replanting corn.
"Before you consider replanting to soybeans, it is imperative that you determine whether a previously applied corn herbicide or herbicides does not restrict your replanting options," he adds.